Radio frequency transmissions are often subjected to multipath fading. Signal blockages at receivers can occur due to physical obstructions between a transmitter and the receiver or service outages. For example, mobile receivers encounter physical obstructions when they pass through tunnels or travel near buildings or trees that impede line of sight (LOS) signal reception. Service outages can occur, on the other hand, when noise or cancellations of multipath signal reflections are sufficiently high with respect to the desired signal.
Communication systems can incorporate two or more transmission channels for transmitting the same program or data to mitigate the undesirable effects of fading or multipath. For example, a time diversity communication system delays the transmission of program material on one transmission channel by a selected time interval with respect to the transmission of the same program material on a second transmission channel. The duration of the time interval is determined by the duration of the service outage to be avoided. The non-delayed channel is delayed at the receiver so that the two channels can be combined, or the program material in the two channels selected, via suitable receiver circuitry. One such time diversity system is a digital broadcast system (DBS) employing two satellite transmission channels.
With reference to FIG. 1, a DBS 10 with time diversity is shown. An uplink facility comprises a splitter 12 for providing multiple channel time division multiplexed (TDM) content 11 to each of two transmission channels 14 and 16. The first transmission channel 14 is transmitted to a first satellite 20 at a first frequency f1 via uplink components indicated at 18. The second transmission channel 16 is delayed by a selected time interval, as indicated at 22, prior to being transmitted to a second satellite 24 at a second frequency f2 via uplink components indicated at 26. A dual arm receiver receives the early and late signals from the satellites 20 and 24, respectively, at a downconverter 28. A delay unit 30 delays the early signal from the satellite 20 via a time interval corresponding to the time interval used to delay the second transmission channel at the transmitter. The delay is applied to all of the channels in the multiple channel TDM content 11. The delayed output from the delay unit 30 can then be synchronized with the late signal and combined, as indicated at 32. A channel selector 34 extracts content corresponding to a particular one of the channels in the multiple channel TDM content in response to a user input, for example.
In a particular implementation of a DBS with time diversity, three satellites 20, 24 and 36 operate in respective ones of tundra orbits 50, 52 and 54, as illustrated in FIG. 2. In other words, the satellites 20, 24 and 36 are in respective ones of three inclined, elliptical orbits which are each separated by approximately 120 degrees. The combination of the 120 degree separation and the rotation of the earth yields a common ground track 60 for all three orbits which is illustrated in FIG. 3. In addition to an approximately 120 degree spatial separation, the orbits 50, 52 and 54 are temporally separated by T/3 or one-third of an orbit period T (e.g., one-third or eight hours of a 24 hour geosynchronous orbit).
With continued reference to FIG. 3, the satellite ground track 60 is a figure-eight, having a northern loop 62 that is smaller than the southern loop 64. The northern and southern loops 62 and 64 share a common ground track point hereinafter referred to as the crossover point 66, as shown in FIG. 4. At the crossover point, satellites descending from the northern loop 62 to the southern loop 64 have the same orbital position as satellites ascending from the southern loop 64 to the northern loop 62. Each satellite 20, 24 and 36 spends approximately one-third (e.g., eight hours) of its orbit time south of the equator 68 and, correspondingly, two-thirds (e.g., sixteen hours) of its orbit time north of the equator. Thus, when one satellite 20 is at perigee, as shown in FIG. 5, the subsatellite points of the other two satellites 24 and 36 cross paths and are therefore in the same sky position at the crossover point 66.
As indicated in FIG. 6, when one satellite 36 is at apogee, the other two satellites 20 and 24 are at essentially equal latitude near the equator 68. Of these two satellites, (e.g., satellites 20 and 24 in FIG. 6), one satellite 20 appears to be rising in the southeast, while the other satellite 24 appears to be setting in the southwest. The rising satellite commences transmitting, while the setting satellite ceases transmitting to comply with international coordination and interference concerns with respect to the allocation of bandwidth for satellite operations. By symmetry of the elliptical orbit, this situation of two satellites at nearly the same latitude occurs halfway through an orbit following the time of perigee, that is, at time T/2 (e.g., 24/2 or 12 hours) past perigee.
In a time diversity system as described above in connection with FIG. 1, the satellites 20, 24 and 36 operate as either the “early” satellite (i.e., the satellite transmitting the nondelayed channel 14) or the “late” satellite (i.e., the satellite transmitting the delayed channel 16), depending on the position of the satellite along the satellite ground track 60. For example, when the satellites 20, 24 and 36 are located along the ground track 60 as depicted in FIG. 6, the satellite 20 is the late satellite for illustrative purposes and is switched on shortly after it ascends past the equator along the southern loop 64. Correspondingly, the satellite 24 is switched off for its travel along the portion of the southern loop 64 that is below the equator 68. The satellite 36 is the early satellite.
When each satellite commences its ascent north of the equator along the southern loop 64, the satellite is switched from “early” to “late”, or “late” to “early”, depending on its “early” or “late” status during its traverse of the previous northern loop 62. Thus, the “early” or “late” status of a satellite changes in an alternate manner after the completion of the period during which the satellite is switched off, that is, while traversing the southern loop 64 when the orbital position of the satellite is at a latitude below the equator 68. Accordingly, in the previous example, when the late satellite 36 reaches a latitude near the equator while descending in the southern loop 64, the early satellite 20 is at apogee, and the satellite 24 is switched on and is commencing its ascent above the equator, approximately eight hours later. The satellite 36 is therefore switched off and the satellite 24 is the late satellite. The uplink components 18 and 26 are each controlled using data from a telemetry, tracking and command (TTC) system 27 which monitors and controls the flight operations of the satellites 20, 24 and 36, as shown in FIG. 1. In accordance with this TTC system data, the uplink components 18 and 26 are commanded to transmit the content on the transmission channels 14 and 16, respectively, to the selected ones of the satellites, depending on their orbital positions and corresponding positions along the ground track 60. Each satellite is capable of receiving either of the frequencies corresponding to the late or early satellite signals as commanded by the TTC system.
In view of the above-described system for operating early and late satellites in tundra orbits, a compromise exists between the elevation angle and the availability of spatial and/or time diversity. When elevation angles to one or two satellites are greatest, at least one method of diversity is less available. This tradeoff situation is presented every T/3 or eight hours where T is a 24 hour orbit period. For example, in the crossover situation depicted in FIG. 5, one satellite 20 is at perigee and is not visible from locations in the United States. The other two satellites 24 and 36 are in essentially the same position in the sky. No spatial diversity is available at such orbital positions for approximately one hour, although time diversity is available. In the switchover situation depicted in FIG. 6, two satellites have nearly the same elevation angle, but different azimuths. The elevation angle for these rising and setting satellites 20 and 24, respectively, is nearly as small as the minimum elevation angle for any satellite visible at that location during the orbit period. The elevation angle of the third satellite 36, however, is the greatest elevation angle for that United States location. Since the setting and rising satellites 24 and 20 are relatively low with respect to the horizon, the rising satellite that is switched on is likely to be obscured by terrestrial obstruction. Thus, a reduced availability of spatial and time diversity exists at such times. This situation exists for approximately one hour and occurs approximately every eight hours. For places in the eastern United States, this situation begins prior to the switchover described with reference to FIG. 6, whereas the situation commences after switchover for places in the western United States.
The tradeoff situations described above emphasize the importance of time diversity. The receiver, as stated previously, stores all of the channels in the multi-channel TDM content signal 11 for a selected period of time. Thus, if both of the satellites are obstructed momentarily, the signal 11 can be recovered from the delayed portion of early received signal. Additionally, since the output of the signal combiner 32 contains the combined early and late signals from all of the channels, the user may change the channel selector 34 and immediately receive the new channel contents from the combined TDM signal. Such storage, however, requires significant memory which increases the cost of the receiver. A need therefore exists for a satellite broadcast system which reduces the memory requirements of the receiver in a time diversity satellite broadcast system. A need also exists for a satellite broadcast system that selectively switches signals transmitted from satellites in selected tundra orbit positions to improve reception of the signals (e.g., by increasing elevation angle).